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My guess is your getting nock end contact producing a false weak. Reducing brace hight moves your arrow tip further out from center and should produce a stiffer acting shaft.


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The Mad Scientist
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Reducing brace hight moves your arrow tip further out from center and should produce a stiffer acting shaft.
Does this really work this way, and if so how? I played around with centershot and couldn't make the arrow act differently. Since spine is really arrow vibration the amount of bend in the arrow doesn't change the frequency, at least that's the theory.

Lowering the brace height reduces the amount of time the arrow is free from the string to do its own thing which affects spine too?
 

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When you shorten brace height you increase speed of the bow. It all has to do with the power stroke . An (example) would be a bow with a 9" brace height and 28" draw would have a 19" power stroke. Versus the same bow set to an 8" brace height would have 20" inches of power stroke. More power stroke equates to a higher fps due to more energy being put into the arrow. The down side maybe less forgiveness, a louder bow, and it will change how the arrow reacts. As in your case weak

Hope this helps

Mike
 

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Discussion Starter · #6 ·
When you shorten brace height you increase speed of the bow. It all has to do with the power stroke . An (example) would be a bow with a 9" brace height and 28" draw would have a 19" power stroke. Versus the same bow set to an 8" brace height would have 20" inches of power stroke. More power stroke equates to a higher fps due to more energy being put into the arrow. The down side maybe less forgiveness, a louder bow, and it will change how the arrow reacts. As in your case weak

Hope this helps

Mike
". . . change how the arrow reacts. As in your case weak." I'm down with less forgiveness and a louder bow, but the "weak" part seems the opposite result to expect from that change. Maybe it's the false read that a couple of folks suggested up a few comments?
 

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Does this really work this way, and if so how? I played around with centershot and couldn't make the arrow act differently. Since spine is really arrow vibration the amount of bend in the arrow doesn't change the frequency, at least that's the theory.

Lowering the brace height reduces the amount of time the arrow is free from the string to do its own thing which affects spine too?
Mat

Are you bare shaft tuning? If so it's pretty easy to move your bare shaft (or fletched shaft) impact point around by changing your center shot. I don't care what the effect is on the static spine (I suspect nothing) all I care about is the impact point. The impact point is easy to effect by center shot.

Matt

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Like I said by lowering the brace height you have increased your bow speed as much 8fps. Thus making your arrows to the weak side. Now if you lower your point weight it will stiffen the arrow bringing your arrow back to center.
". . . change how the arrow reacts. As in your case weak." I'm down with less forgiveness and a louder bow, but the "weak" part seems the opposite result to expect from that change. Maybe it's the false read that a couple of folks suggested up a few comments?
 

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In my experience the reality is that actually different bows react to changes in BH in different ways due to a few different factors. Where were you in the BH range before the change? Where does the change put you now? Where were you in state of tune? Edge of weak? Edge of stiff? Blah blah blah. So really what we have in regards to BH change more a general rule of thumb. Lower the BH and the arrow will try and behave as if it is stiffer. (Of course it isn't actually stiffer, it is what it is.) Anyway the change comes about because as they say, "Timing is everything." Lower the BH, despite the added KE the arrow is free from the string later, and the nock end is approaching the riser later in the power stroke. And that change happens regardless of what changes may happen to amplitude and frequency of the arrow oscillations.

So in regards to changes in BH and subsequent changes of state of tune, you note where you're starting, make the changes, then note what the change did for that particular current set up. And when doing this, beware the false weak indication, it is a VERY distinct possibility when lowering BH.
 

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Here is a pretty good image of what's going on.



You get low enough and your nock end will wack the riser an flip the shaft to the right.

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The Mad Scientist
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I do the bare shaft nock left/right to determine spine. The center shot does not matter to spine, and since I don’t have sights I don’t have a good measure of impact changes. I set BH with my chronograph, there is a definite sweet spot of max velocity, under or above this arrow speed goes down. While the power stroke is longer with a short BH, the draw weight is less. The opposite is true with a longer BH. I’m guessing that setting BH by noise levels is the same thing, less efficiency (speed) = more noise.

Matt’s diagram is a good illustration but it ignores the string/arrow side to side movement (paradox). There was a post on AT about this, it’s really the timing of the string/nock separation that determines spine. That’s the only explanation for the left/right bare shaft reaction. Changing the BH would affect this. I’ve “weakened” arrows by increasing the BH w/o any measureable loss in speed, so it was all a nock separation timing event change.
 

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Every time this topic has been debated on a (primarily hunting) site I don't frequent as much anymore, the opinions consistently ran about 50% whether a lower BH would make an arrow act stiffer/weaker and why. Folks here seem to be more interested in and knowlegeable about the technical aspects of archery and I expected there'd be more of a consensus. Doesn't look that way, but I do think a technical tuning resource linked here is in the "nock coming off the string later" camp. I'm just going to get my popcorn and keep watching, lol.
 

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Discussion Starter · #15 ·
Well, I appreciate all responses and olddogrib's take on the lack of consensus. Actually, changed shelf and strike plate surfaces last night after noticing some degrading of the shelf surface. Realized that in doing so I had effectively moved the strike plate out a little, so I wanted to shoot same arrows with same brace height today and compare results. Sure enough, arrows that showed a little weak yesterday showed right on/ever so slightly stiff today through paper. After shooting those down range a few minutes ago they really fly and impact straight when I do my part. So stopping right here for now. Again, thanks for all responses.
 

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Well, I appreciate all responses and olddogrib's take on the lack of consensus. Actually, changed shelf and strike plate surfaces last night after noticing some degrading of the shelf surface. Realized that in doing so I had effectively moved the strike plate out a little, so I wanted to shoot same arrows with same brace height today and compare results. Sure enough, arrows that showed a little weak yesterday showed right on/ever so slightly stiff today through paper. After shooting those down range a few minutes ago they really fly and impact straight when I do my part. So stopping right here for now. Again, thanks for all responses.
Cool - if your happy with your tune WRITE IT DOWN !!!!!!

LOL - been there done that

Matt
 

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The center shot does not matter to spine, and since I don't have sights I don't have a good measure of impact changes.
Like I said the arrow is what it is. But EVERYTHING we do affects how the arrow actually behaves. Shoot a self bow that effectively has absolutely no centershot whatsoever. You'll rethink the relationship between center shot and spine for sure.
 

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I bought the iphone app appitune by Jake kaminski yesterday. None of the pieces were new to me but his process of tuning is interesting. In coming to a full tune he bare shafts, walk back tunes and group tunes but, he never mentions paper tuning.

He does however say to make a shaft act weaker you increase brace and to make it act stiffer you decrease brace.

Matt


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I like group tunning for form and to check all my arrows to make sure I don't have a flyer. Then I walk back tune from 30yrd first back 5yrds at a time to 15 yrds for my line left and right.

Mike
I bought the iphone app appitune by Jake kaminski yesterday. None of the pieces were new to me but his process of tuning is interesting. In coming to a full tune he bare shafts, walk back tunes and group tunes but, he never mentions paper tuning.

Matt

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He does however say to make a shaft act weaker you increase brace and to make it act stiffer you decrease brace.

Matt

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I must be reading this backwards Matt. If you increase BH you basically weaken the bow so wouldn't that make your shaft act stiffer?

Sent from my iPhone using Tapatalk
 
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